Components of various industrial devices are often subjected to extreme conditions, such as high impact contact with abrasive surfaces. For example, such extreme conditions are commonly encountered during subterranean drilling for oil extraction or mining purposes. Device components in these conditions have a longer usable life it they are coated with a layer of diamond to form a polycrystalline diamond (PCD) layer on the component's surface. Diamond is extremely hard, conducts heat away from the point of contact with the abrasive surface, and may provide other benefits to the component. Device components coated in some fashion with polycrystalline diamond are generally referred to as PCD elements. They are sometimes also referred to as PCD compacts.
PCD elements are formed by covering the surface of an substrate with grains of diamond crystal and a catalyst material, then subjecting the covered surface to high temperature and pressure sufficient to cause the diamond grains to fuse into a diamond body matrix in a PCD layer. The catalyst also becomes trapped in the PCD layer in an interstitial matrix. The interstitial matrix is typically continuous within the diamond body matrix.
The catalyst trapped in the PCD layer both provides additional benefits as compared to diamond alone and causes certain detriments. For example, by filling in what would otherwise be empty space in the PCD layer, the catalyst provides structural integrity and makes the PCD layer stronger. For example, the ability of the PCD layer to resist shock loading, sometimes called impact strength, is improved by the presence of the catalyst. One patent, U.S. Pat. No. 4,224,380 to Bovenkerk, reports a decrease in strength of 20% between leached and unleached PCD layers. However, because the catalyst is not as hard as diamond, it reduces the ability of the PCD layer to not be worn down by the surfaces it contact, which is often referred to as abrasion resistance. Similarly, because the catalyst does not conduct heat as well as diamond, it heats up near the point of contact with the abrasive surface much more rapidly than does the diamond. This tendency can cause the catalyst to swell and cause small cracks in the surrounding PCD layer or result in other physical damage. Thus, the catalyst reduces the thermal stability, or ability to withstand degradation due to high temperatures, of the PCD layer.
As a result of these or other effects, it is sometimes advantageous to remove the catalyst from all or part of the PCD layer. The most common process for catalyst removal uses a strong acid bath, although other processes that employ electrolytic and liquid metal techniques exist. When using an acid bath process, two regions of the PCD element are typically protected. First, any non-PCD portions of the element, such as the substrate, are protected. These portions will typically be damaged or may detach from the PCD layer if they are exposed to acid. Second, any PCD portion that will not be leached is protected. Due the ability of leaching to cause both positive and negative effects, the areas of a PCD element leached as well as other effects of the leaching process may be varied to achieve different effects.
Leaching is typically performed in batches, with several PCD elements being leached at once. The leaching process, like most industrial processes, is carefully controlled such that the amount of catalyst removed, the depth from the PCD layer surface to which it is removed, and other effects tend to be very similar from batch to batch. Many properties of a leached PCD element can often only be accurately determined through destructive testing. Accordingly, it is a common practice to evaluate the effects of leaching and the resulting PCD element properties by selecting one or more representative PCD elements from each batch, or periodically as representative of multiple batches, and performing testing on those PCD elements.
Although various types of leached PCD elements exist, many more remain to be developed and evaluated. In particular, PCD elements that balance the benefits and disadvantages of leaching to improve or maximize the usable life of the PCD elements or devices containing them are needed.